JOU RNAL OF RESEARCH of the National Bureau of Standards - A. Physics and Chemistry Vol. 71 A, No.5, September-October 1967 . Dissociation Constants of Some Substituted Nitrophenols In
Aqueous Solution at 2S 0 C
R. A. Robinson
Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234
(May 9, 1967)
The dissociation constants of twelve substituted phenols with a nitro group in the 0- or p-position have been determined by spectrophotometric measurements in aqueous solution at 25°C.
Key Words: Dissociation constant, nitrophenols, substituted phenols.
1. Introduction 2,4-Dinitro-6-methylphenol was recrystalli2;p.d twice from benzene-cyclohexane mp 86.0-86.7 °C; 2,6-dini The determination of the dissociation constants of tro-4-methylphenol from benzene-cyclohexane and tnen a number of substituted phenols by the spectrophoto from cyclohexane, mp 80.7- 81.2 °C; 2,4-dinitro-3- metric method has been described recently [1, 2, 3, 4, methyl-6-isopropylphenol from aqueous ethanol and 5, 6, 7).1 The purpose of this paper is to present data th en from li groin , mp 55.0-55_7 °C. 2,4-Dinitro-6- for twelve substituted phenols with an indicator phenyl phenol recrystallized from methyl ethyl ketone range pH 2.1-5.5. and th en from ethyl acetate, mp 206.6-207.0 °C; 2,4-dinitro-6-isopropylphenol from 95 percent ethanol and then from hexane, mp 54.0-55.0 0c. 2. Experimental Procedure Table 1 gives some characteristics of the absorption spectra of these compounds, recording the wavelengths 2.1 . Materials and extinction coeffi cients at maximum absorption in both acid and alkaline solution, and also those at the With the exception of 2,6-diiodo-4-nitrophenol, the isosbestic points. Figure 1 shows the absorption spec materials were commercial products. I am indebted trum of one of these phenols, 4-chloro-2,6-dinitro-3- to Dr. Marion M. Davis for the following details of methyl phenol, in alkaline solution where the phenol their purification. is present in the form of the phenolate anion, in 2-Chloro-4, 6-dinitrophenol was recrystallized twice strongly acid solution where it exists as the undissoci from benzene-cyclohexane, mp 111.0- 111.5 0c. 4- ated molecule, and in a dilute solution of hydro Chloro-2,6-dinitro-3-methylphenol was recrystallized chloric acid were both species are present; the isos once from benzene-cyclohexane and then from li bestic point is at 378 fl1I.Lwith an extinction coefficient groin, mp 68.0-68.5 °C; 2-chloro-6-nitrophenol first E=2,740. from 95 percent ethanol and then from benzene cyciohexane, mp 70.0-70.5 °C . 2,4-Dibromo-6-nitro TABLE 1. Characteristics of the absorption spectra of substituted phenol was purified in the same way, mp 117.0-117.5 phenoLs in aqueous soLution at 25 °C. 0c. 2,6-Dibromo-4-nitrophenol was recrystallized three times from benzene-cyclohexane, then from aqueous Alkaline Acid Isosbesti c ethanol and then from benzene-cyclohexane, mp solutio n solution poi nt E:2 tnJJ; IllM tnI' 141.8-142.0 °C, 2,6-dichloro-4-nitrophenol twi ce from .. • . benzene-cyclohexane, mp 125 °C (dec.). 2,6-Diiodo-4- 2-C hloro-4,6-din it rophe nol. _.. 14.910 370 10.100 268 4.250 326 4-C hloro.2,6.dinitro-3· me thylphenol .. 5.660 425 3,950 355 2.740 378 nitro phenol was synthesized by the method of Datta 2·Chloro·6·nitrophenol .. 4.580 420 2,620 350 2.200 372 and Prosad [8] by adding p-nitrophenol in an excess 2,4· Dibromo·6·n it rophenol ...... , ... 4.1 80 430 2.370 365 2. 11 0 383 2,6· Dibl'omo·4·n it rophenol .... 15,720 402 7.650 312 3.900 344 " of ammonia to a solution of iodine in aqueous potas 2,6· Die hloro-4·n it rophenol .. 13,850 399 7.020 312 3.500 343 2,6· Diiodo-4-n it rophenol ...... 15.620 409 6.550 318 3.570 ·35 1 sium iodide, recrystallized from aqueous ethanol and 2,4- Dinitro·6·meth ylphenol. .. 14.2 10 377 13.630 'l.'72 5.330 333 2.6·Dinitro.4.merh ylphenol...... 7.250 450 5.100 365 2.390 393 then from benzene-cyclohexane, mp 156.5-157.0 °C. 2,4-Dinitro·3·meth yl-6·isopropylphenol ... 14.630 396 5.500 300 3.900 335 2,4-Di nitro.6.phenylphenol .. 14.990 380 8,290 265 4.280 334 2,4-Din i tro·6-isopropyl phenol .. 13.400 375 5.680 265 5.220 335 I Figures in brackets indicale the literature references aJ the end of this paper.
385 TABLE 3. Dissociation constant of 4-chloro-2,6-dinitro-3-methyl 425 m}" phenol in aqueous solution at 2S °C 6,000 E2 =5,660 - a.Olm- NoOH ---- tm- Her 5,000 355m}" ...... 0.0012 m - Her Concentration of phenol: 2.6 X 10- 5 M, cell length 4 cm, wavelength . ,03,950 425 mJL , D , 0.012 , D2 0.583 4,000 /--, / , E / , 3,000 /1 ...... ~ mHCI X 10' D "'H + X 10' - 2 log Y pK / . / \~ ~8r~ }", . 02,740 2,000 "".. ;"' ...... -/ ..... ', ...... \ \ 2.192 0.420 2.378 0.016 3.241 1,000 " 6.048 .293 6.176 .025 3.248 " 8.084 .257 8 .196 .029 3.239 300 350 400 450 500 550 10.28 .228 10.38 .032 3.232 WAVELE NGT H (m}") 12.06 .205 12.15 .034 3.241
FIGURE 1. Absorption spectra of 4-chloro·2,6-dinitro-3-methyl Average ...... , 3,240 phenol,
2.2. Method
The spectrophotometric method of determining the dissociation constant of a weak acid has been described TABLE 4, Dissociation constant of2-chloro-6-nitrophenol in aqueous many times [1, 2, 3, 4, 5, 6, 7], For the five phenols solution at 2S °C with pK values less than 3_5, dilute solutions of hydro chloric acid were found to be suitable media in which to make measurements; for the other seven, with Concentration of phenol: 3 x 10- 5 M, cell length 4 cm, wavelength pK values greater than 3,8, succinate buffers were 430 mJL , D, 0,014, D2 0,538 Buffer solution: Sodium hydrogen succinate and disodium succinate, U!;;ed_ In some cases a small correction was needed each at molality m for the effect of the phenol on the acidity of the buffer solution; this correction has already been described [9], m D p(aHYcl) pK pK (co rr,)
3. Results and Discussion 0,0100 0,294 5,553 5,493 5,491 ,0175 ,290 5,532 5,485 5,483 Details of the spectrophotometric measurements ,0250 .285 5,511 5,481 5,480 are given in tables 2 to 13, The results are summarized ,0375 .281 5.499 5,482 5,481 in table 14. ,0500 .274 5,477 5,483 5,483 It is known that ortlto and para substitution of a nitro
group in phenol have effects which are by no means Average ...... , .. , .. , ...... 5.483 additive, Thus, if the pK value of phenol itself is 9_99 8 [10], that of o-nitrophenol 7,230 [11], and that of p-nitrophenol 7,156 [12], then 0- and p-nitro substitu tion lowers the pK \' alue , of phenol by 2,768 and 2_842, respectively, We might, therefore, expect pK = (9_998 - 2.768 - 2.842 )= 4,388 for 2,4-dinitrophenol, whereas TABLE 5. Dissociation con!tant of 2,4-dibromo-6-nitrophenol m aqueous solution at 2S °C
TABLE 2, Dissociation constant 0/ 2-chloro·4,6-dinitrophenol Ln aqueous solution at 2S °C Concentration of phenol: 4.5 X 10- 5 M, cell length 4 cm, wavelength 430 mJL, D, 0.067, D2 0.757 Buffer solution: sodium hydrogen succinate and sodium chloride, Concentration of phenol: 1.18 X 10- 5 M, cell length 4 cm, wavelength each at molali t y m 372 mJL , D, 0.073, D, 0.660
m D p (aHYcl) pK pK (corr.) mHCI X 10" D mH+ X 103 -2 log Y pK
2.210 0.542 2.221 0.046 2.100 0.02 0.467 4.853 4.713 4.712 3.206 .508 3.216 .054 2.090 .04 ,461 4,833 4.709 4,708 4.967 .454 4.976 .065 2.101 .06 .458 4.821 4.704 4.704 7.389 .403 7.397 .081 2.103 .08 .454 4.811 4.705 4,705 10.05 .359 10.06 .089 2.108 .10 .449 4.802 4.709 4.709
Average ...... ' " 2.100 Average ...... , 4.708
386 TABLE 6. Dissociation constant of 2,6-dibromo-4-nitrophenol in TABLE 9. Dissociation constant of 2,4-dinitro-6-methylphenol in aqueous solution at 25°C aqueous solution at 25 °C
5 Concentrati on of phenol: 2.05 x 10- M, cell length 4 cm, wavelength Co ncentration of p.henol: 8.3 X 10- 5 M, cell length 1 cm, wavelength 402 mp.- , D, 0.005 , Dz 1.274 400 mp.-, D, 0.028, Dz 0.969 Buffer solution: sodium hydrogen succinate and sodium chloride each at molality m mHC' X 10' D II'H + X 10' - 2 log 'Y pK
m D p(aH'YCI) pK pK (corr.) 2.013 0.855 2.150 0.01 5 3.376 3.106 .726 3.222 .018 3.391 4.018 .649 4.122 .020 3.392 0.02 0.691 4.853 4.476 4.471 4.980 .584 5.073 .023 3.394 .03 .688 4.840 4.469 4.466 6.057 .527 6.141 .025 3.393 .05 .680 4.826 4.473 4.471 6.999 .482 7.076 .027 3.397 .07 .675 4.816 4.474 4.473 8.015 .447 8.086 .028 3.392 .10 .669 4.802 4,472 4.471 9.003 .415 9.069 .030 3.393 9.996 .385 10.06 .03J 3.397 Average...... 4.410
Average ..... "...... 3.392
TABLE 7. Dissociation constant of 2,6-dichloro-4-nitrophenol in aqueous solution at 25 °C TABLE 10. Dissociation collstant of 2,6-dillitro·4-meth ylphellol in. aqueous solution at 25 °C
Concentration of phenol: 2.4 X 10- 5 M, cell length 4 cm, wavelength 400 mp.-, D, 0.002, Dz 1.340 Concentration of phenol: 2.25 X 10- 5 M, cell length 4 cm, wavelength 450 mp.- , D, 0, Dz 0.650 tnHCI X 104 D mH +X 104 - 2 10g 'Y pK Buffer so lution: mixture of sodium hydrogen succinate (m) and hydrochl oric acid (0.6667m) equivalent to a mixture of sodium hy· drogen sll ccinate (0.3333m), succini c acid (0.6667m), and sodium 1.987 0.792 2.129 0.015 3.528 c hl oride (0.6667m) 2.989 .655 3.106 .018 3.547 4.033 .562 4.133 .020 3.548 5.008 .497 5.097 .023 3.547 m D p(aH'YCI) pK" 6.314 .426 6.390 .025 3.553 6.583 .415 6.657 .026 3.553 7.786 .370 7.852 .028 3.554 0.02 0.206 3.898 4.232 8.773 .340 8.834 .030 3.555 .04 .203 3.887 4.230 9.973 .308 10.03 .031 3.558 .06 .202 3.882 4.228 .08 .199 3.878 4.233 .10 .199 3.875 4.230 Average...... 3.549 Average ...... , ...... 4.23,
TABLE 8. Dissociation constant of 2,6-diiodo-4-nitrophenol in " The correction to pK is negligible. aqueous solution at 25 °C
Co ncentration of phenol: 2.5 X 10 5 M, ce ll length 4 C lll , wavelength 409 mp.- . D, 0.005. D2 1. 540 'the observed value is 4.09 [13]. In the same way,
mHCI X 10' D m + X 10' -2 log 'Y pK pK = 4.46z might be predicted for 2,6·dinitrophenol, H compared with the observed value of 3.71 [14]. J An additivity principle works much better with 2.019 1.065 2.192 0.015 3.324 meta substitution; pK = 8.355 has been found [15] for 2.975 0.946 3. 128 .018 3.323 3.861 .864 4.001 .020 3.314 m·nitrophenol and hence one would predict pK 6.71 4.144 .823 4.277 .020 3.332 for 3,5·dinitrophenol, compared with the observed 4.965 .764 5.089 .023 3.326 value of 6.69 [1]. 6.063 .691 6.175 .025 3.327 7.791 .614 7.890 .028 3.313 The substitution of a methyl group in the para posi ,. 8.959 .557 9.059 .030 3.324 tion to give p·cresol raises the pK value of phenol by \ 9.984 .513 10.07 .032 3.335 0.264 [16]; pK = 7.494 might be predicted for 4- methyl·2·nitrophenol compared with the observed Average ...... 3.32. value of 7.597 [6]. The difference betwtien the ob· served and calculated value is not large, but the 387 observed value is higher than the calculated, whereas TABLE 13. Dissociation constant of 2,4-dinitro-6-isopropylphenol for the dinitrophenols mentioned above the observed in aqueous solution at 25°C value was lower than the calculated. A quantitative explanation of this anomaly, the difference between Concentration of phenol: 9.6 X 10- 5 M, cell length 1 cm, wavelength the observed and calculated pK values, is not yet 400 m,.. D, 0.029, D, 0.791 possible, but the change in sign of the difference in Buffer solution: sodium hydrogen succinate and sodium chloride each at molality m. going from 4-methyl-2-nitrophenol to 2,4- or 2,6- dinitrophenol suggests that the anomaly increases with a decrease in the pK value of the phenol into m D p(aHYc,} pK pK (corr.) which an 0- or p-nitro group is introduced. 0.02 0.541 4.853 4.542 4.536 .03 .539 4.840 4.534 4.530 .05 .529 4.826 4.545 4.543 .07 .528 4.816 4.538 4.536 TARLE 11. Dissociation constant of 2,4·dinitro-3-methyl-6-iso .10 .524 4.802 4.534 4.533 propylphenol in aqueous solution at 25°C
Average ...... 4.536 Concentration of phenol: 1.09 X 10- 4 M, cell length 1 cm, wavelength 396 m,.. , D, 0.032, D, 1.594 Buffer solution: mixture of sodium hydrogen succinate (m) and hydrochloric acid (0.6667m) equivalent to a mixture of sodium TABLE 14. Summary of the pK values of substituted phenols in hydrogen succinate (0.3333m), succinic acid (0.6667m) , and aqueous solution at 25 °C sodium chloride (0.6667m) pK 2-C hloro-4,6·dinitrophenol...... 2.100 m D p(aHYc,} pK pK (corL) 4-Chloro-2,6·dinitro-3-methylphenol...... 3.240 2-C hloro-6-nitrophenol...... 5.483 2,4·Dibromo·6·nitrophenol...... 4.70. 0.02 0.758 3.898 3.959 3.954 2,6·Dibromo·4·nitrophenol...... 3.392 .03 .752 3.892 3.960 3.957 2,6· Dichloro-4· nitrophenol...... 3.54• .04 . 749 3.887 3.958 3.955 2,6·Diiodo-4-nitrophenol...... 3.32 • .05 .746 3.884 3.959 3.957 2,4·Dinitro-6·methylphenol...... 4.470 .06 .743 3.882 3.961 3.959 2,6-Dinitro-4·methylphenol...... 4.23, .08 . 738 3.878 3.961 3.960 2,4-Dinitro-3-methyl-6-isopropylphenol...... 3.95 • .10 . 735 3.875 3.962 3.961 2,4-Dinitro-6-phenylphenol...... 3.84• 2,4-Dinitro-6-isopropylphenoJ...... 4.536
Average ...... ,...... 3.958 The available literature values have been used to study this possibility. Data are available for p-nitro substitution in the following compounds, pK values being known for the phenols before and after sub- TABLE 12. Dissociation constant of 2,4-dinitro-6-phenylphenol!ll stitution: aqueous solution at 25°C
Concentration of phenol: 1.79 X 10- 5 M, cell length 4 cm, wavelength pK 380 mj.L , D, 0.142, D, 1.070 pK Nitro derivatives Buffer solution: mixture of sodium hydrogen succinate (m) and hydro Parent phenols Parent Differ- . chloric acid (0.6667m) equivalent to a mixture of sodium hydrogen phenols ences succinate (0.3333 m) , succinic acid (0.6667m), and sodium chlo Pre- Observed ride (0.6667m). dieted
o-Chlorophenol...... 8.53. [10] 5. 69, 5.45 [I7]i 0.24 m D pK pK (corL ) P(OHYCI} o-Nitrophenol...... 7.230 [ll] 4.38. 4.09 [13] 0.30 2,6-Diehlorophenol...... 6.79, [4]. 3. 949 3.54. 0.40 2,6-Dibromophenol...... 6.67, [7] 3.83, 3.39, 0.44 0.01 0.634 3.906 3.854 3.852 2-C hloro-6- ni trop henol .. . 5.483 2.64, 2.10 0.54 .02 .629 3.855 3.851 0 3.898 2,6-Dinitrophenol...... 3.71 [14] 0.87 0.33 [18] 0.54 .03 .627 3.892 3.853 3.852 .04 .626 3.887 3.850 3.849 .05 .628 3.884 3.843 3.843 .06 .627 3.882 3.843 3.843 .07 .627 3.880 3.841 3.841 Thus, o-chlorophenol, with a pK value of 8.534 , can be .08 .620 3.878 3.852 3.852 converted to 2-chloro-4-nitrophenol, whose predicted .09 .620 3.877 3.851 3.851 pK value (PK 5.692) is 2.842 lower, the observed value being 5.45 and the anomaly 0.24. The absence of a Average ...... 3.84. literature reference in the above table means that the datum was taken from the present paper. 388 For o-nitro substitution the following data hav{ into which the nitro group is introduced. It can be been found, assuming 2_76s for the effect of the nitro seen that, in general, the anomaly increases · in mag group: nitude as the acid strength of the phenol increases (PK decreases). For p-nitro substitution, the value for the formation of picric acid from 2,6·dinitrophenol pK nitro derivatives Parent phenols pK parent Differ- seems somewhat small. phenols ences Predicted Observed For o-nitro substitution, the anomaly seems to be uniformly greater as the pK value of the phenol de creases but there are three peculiar cases, the forma p-Cresol ...... 10.26" [16] 7.49. 7.59, [6] -0.10 p-Chlorophenol.... 9.418 [10] 4.388 4.09 [13] 0.30 tion of 2,6-dinitrophenol from o-nitrophenol, that of o-C hlorophenol. .. 8.53. [11] 5.77. 5.483 0.29 4-chloro-2,6-dinitrophenol from 4-chloro-2-nitrophenol, 2,4·Dibromo· 7.790 [7] 5.02, 4.70, 0.31 and that of picric acid from 2,4-dinitrophenol. In each phenol. case the anomaly seems to be somewhat large. It o-Nitrophenol.. ... 7.230 [11] 4.46, 3.71 [14] 0.75 p-Nitrophenol. .. .. 7. 15. [12] 4.38, 4.09 [13] 0.30 should be noted that in each case the synthetic. proc 4·Chloro·2· 6.46 [17] 3.69 2.96. [5] 0.72 ess is one in which an o-nitrophenol is converted into nitrophenol. a 2,6-dinitrophenol, each of which has adjacent 2·Chloro·4· 5.45 [17] 2.68 2.100 0.58 N02-OH-N02 groups. In such cases marked steric nitrophenol. 2,4·Dinitro' 4.09 [13] 1.32 0.33 [18] 0.99 inhibition of resonance would be expected. phenol.
These anomalies, (/lpK = pKcalcd. - PKobSdJ, are plotted in figure 2 against the pK value of the pbenol
1.0 • ----r----,--,----,---,----,----, My sincere thanks are expressed to Dr. Marion M. 2,4- di - N02 Davis for providing the purified chemicals used in this work.
0 .8 4. References
•4 - CI-2-N0 2 [1] R. A. Robinson, M. M. Davis, M. Paabo, and V. E. Bower, 1- Res. NBS 64A, (phys. and Chern.) No.4, 347 (1%0). [2] M. M. Davis, M. Paabo, and R. A. Robinson, J. Res. NBS 64A, 0.6 . 2-CI-4-N02 (phys. and Chern.) No.6, 531 (1960). [3] M. M. Davis and M. Paabo, 1- Res. NBS 64A, (phys. and o o Chern.) No.6, 533 (1960). _ .. lIpK 2,6-di- N02 2- CI- 6 - N02 [4] R. A. Robinson, J. Res. NBS 68A (phys. and Chern.) No.2, ,6-di-Br 159 (1964). [5] E. E. Sager, R. A. Robinson,. and R. G. Bates, 1- Res. NBS 68A 0.4 o (phys. and Chern .) No. 3, 305 (1964). 2,6-di-CI [6] R. A. Robinson and R. G. Bates, J. Res. NBS 70A (phys. 2,4- di-Br and Chern.) No. 6, 553 (1966). Ii [7] R. A. Robinson, J. Res. NBS 11A (phys. and Chern .) No.3, a-or p- N02 ~} o-CI 217-222 (1967). [8] R. L. Datta' and N. Pros ad, J. Am. Chern. Soc. 39,441 (1917). 0.2 .P-CI [9] R. A. Robinson and A. K. Kiang, Trans. Faraday Soc. 51, 1398 (1955). [10] A. I. Biggs and R. A. Robinson, J. Chern. Soc. (London) 388 (1961). [11] R. A. Robinson and A. Peiperi, J. Phys. Chern. 67, 1723 (1963). o [12] G. F. Allen, R. A. Robinson, and V. E. Bower, 1. Phys. Chern. 66, 171 (1962). [13] H. von Halban and G. Kortiim, Z. physik. Chern. A170, 351 (1934). [14J G. Kortum and H. Wifski, Z. physik. o,(,ln. 2, 2"56 -(1954). 3 4 6 7 8 9 10 [15] R. A. Robinson and A. Peiperi, 1- Phys. Chern. 67, 2860 (1963). pK [16] A. I. Biggs, Trans. Faraday Soc. 52,35 (1956). ,. [17] V. E. Bower and R. A. Robinson, 1- Phys. Chern. 64, 1078 FIGURE 2. ~pK =pK (calcd.)-J!K (obsd.), where pK (calcd.) is (1960). derived on the assumption that (1 ) o·nitration of a sub!tituted phenol [18] M. M. Davis and M. Paabo, 1- Res. NBS 67 A (phys. and lowers the pK value by 2.768, identicaT with the lowering of pK Chern.) No.3, 241 (1963). foun.d for the o·nitration of phenol itself; (2) p·nitration of a sub· stituted phenol lowers the pK value by 2.84" identical with the lowerin.g of pK found fo r the p·nitration of phenol itself The lettering within the fi gure indicates the structure of the 'parent substituted phenol which undergoes Q· nilration (e ) or p-nitration (lJ). The abscissa gives the pK value of the parent phenol. (Paper 7lAS-469) 389